WHEN Ronald Reagan stormed into office in a landslide victory in 1980, a large number of Republican senatorial candidates rode in on his coattails--enough to give the GOP control of the Senate until 1986. With a working coalition in the House composed of Republicans and conservative Southern Democrats, Reagan was able to push through much of his administration's conservative agenda despite significant opposition to his policies.

If George Bush defeats Gov. Michael S. Dukakis in the presidential election two weeks from now--as recent polls indicate he would if the election were held today--the vice president will not be able to count on a Republican, or even conservative, Congress to rubberstamp his administration's initiatives. Despite the likelihood of Republican success in the presidential election, the Democratic Party seems sure to make strong gains in the Senate and at least retain its control of the House in the 101st Congress.

The House of Representatives has been controlled by the Democrats since the 1950s, and a Bush victory, no matter how large, will not change that. There are currently 255 Democrats and 177 Republicans in the House. Since better than 90 percent of all members of Congress are re-elected each term, nothing short of an act of God could return the House to the GOP.

For the Republicans, the Senate is more promising than the House, but even there it is unlikely that the GOP will regain a majority. The Democrats currently have 54 seats as compared to the Republicans' 46. There are 33 Senate seats--18 Democrats, 15 Republican--up for grabs this year, and polls indicate that the Democrats will not only hold on to most of the seats they already have, but also should take away some from the GOP.

ONE explanation for the Democrats' likely success is the fact that they have learned how to play hardball with the Republicans in terms of campaign financing. This year, the Democrats have actually raised more money than the Republicans. A report in The New York Times this week showed that as of September 30, Democrats had raised $1 to every 87 cents raised by the Republicans. In 1986, the Republicans garnered $1.35 to every $1 earned by the Democrats.

Another factor working for the Democrats is the stature of its incumbent candidates. Some of the party's best-known leaders are up for re-election, including Majority Leader Robert C. Byrd (W.Va.), Edward M. Kennedy '54 (Ma.) and Daniel Patrick Moynihan (N.Y.). Other Democratic safe bets include Spark M. Matsunaga (Ha.), George Mitchell (Me.), Jim Sasser (Tenn.) and, of course, Lloyd Bentsen (Tex.).

Although it is easier to field viable challengers against incumbent senators than against representatives, the Republicans have not had a very good recruiting year. Most Republican challengers have never served in a statewide office before, while many of the Democratic challengers have done so successfully.

THE Republicans have targeted New Jersey and Ohio as two key states, and have been able to recruit viable candidates in each. The GOP is still hopeful that Pete Dawkins will be able to upset Democratic incumbent Frank Lautenberg in the Garden State despite polls showing Dawkins slightly behind. Dawkins--a Heisman Trophy winner at West Point who became an Army General and later a successful business executive--would seem a mirror image of Sen. Bill Bradley (D-N.J.), who went from Princeton to Oxford to the New York Knicks to the U.S. Senate. But after performing well in early polls, Dawkins has fallen behind due to a flawed campaign organization.

Ohio may also become a win for the GOP, which has tapped Cleveland Mayor George Voinovich to run against incumbent Sen. Howard M. Metzenbaum. Voinovich has raised a lot of money, but his negative campaign ads appear to have hurt him more than helped. Metzenbaum, who has a fairly comfortable lead in the polls, has also had the support of Ohio's popular Gov. Richard Celeste.

THERE are also several Republicans who are sure bets to win re-election, including Orrin Hatch (Ut.), Richard Lugar (Ind.), John C. Danforth (Mo.) and John Heinz (Pa.). The Bush campaign's success in restoring confidence in GOP leadership after Reagan popularity began to diminish has helped some Republican incumbents who would have faced much more serious challenges. Sen. Dave F. Durenberger (R-Minn.) continues to hold onto a fairly strong lead against Hubert H. Humphrey III, the son of the former vice president.

Still, the GOP's incumbent senators will likely lose more than their counterparts on the other side of the aisle. There are at least four states--California, Connecticut, Nebraska and Nevada--where Republican incumbents are still in serious jeopardy.

In California, GOP Sen. Pete Wilson has raised $12 million--more money than any other Senate candidate in the country--but his lead over Lt. Gov. Leo T. McCarthy has not significantly improved. With Wilson's funds starting to dwindle, the race could turn into a bitter publicity battle over the next two weeks. Any misstep by Wilson could make the race a toss-up.

Connecticut's Lowell Weicker, the most liberal Republican in Congress, faces opposition from State Attorney General Joseph Lieberman, a moderate-liberal Democrat backed by a weird combination of political forces. Even National Review Editor William F. Buckley--who can't stand Weicker--supports the Democrat in what may be the best modern example of politics making strange bedfellows. Lieberman is running even with veteran Weicker in the polls, but even if the incumbent loses, conservative Republicans won't be too upset.

Two races which Democratic challengers should win are in Nebraska and Nevada. Sen. David K. Karnes (R-Neb.) who was appointed to the post following the death of Sen. Edward Zorinski, is trailing popular former Gov. Robert Kerrey by a significant margin. And Sen. Chic Hecht (R-Nev.), who was voted one of the least effective senators by Washingtonian magazine, and was considered a re-election long-shot from the start, will probably fall to Democratic Gov. Dick Bryan in a close race.

IN the races for the six open seats, left by three retiring senators from each party, U.S. Rep. James Jeffords (R-Vt.) is expected to move to the upper house, while Democrat Charles S. Robb should win the Virginia seat, formerly held by a Republican. The four other open races--in Florida, Wisconsin, Washington and Mississippi--feature the closest battles.

U.S. Rep. and House Minority Whip Trent Lott (R-Miss.) will likely hold onto a slim lead in Mississippi, and Democrat Herbert Kohl, who has financed his campaign primarily out of his own pockets, will probably get the nod to replace Sen. William Proxmire in Wisconsin. The other two races--between Rep. Connie Mack III (R) and Rep. Buddy MacKay (D) in Florida and between Rep. Mike Lowry (D) and former Sen. Slade Gorton (R) in Washington--are too close to call.

For the Republicans to regain control of the Senate, they must take five of the six open seats, hold onto all their incumbents (including Weicker, Karnes and Hecht) and pick off Lautenberg and Metzenbaum. This will give the GOP a 50-50 tie in the upper house, which could be broken by the vote of Vice President Dan Quayle, assuming that a Republican is elected to fill Quayle's seat in Indiana.

But all that is highly unlikely. Right now it looks like the Democrats will hold off the challenges to their incumbents, beat Hecht and Karnes and at least split the open seats. This result will still increase the Democratic majority in the Senate to 56-44, a margin which will make life much more difficult for a President Bush and his agenda than it was for President Reagan.