Presidential candidates are aiming their final blitz of handshakes and media appearances at undecided voters in the Iowa caucuses. Today Iowans will give them their first decisive grades on the severe test of presidential politics.

After what for some has been years of campaigning, the Iowa caucus will narrow the field of candidates by giving media attention and momentum to the winners, while pushing others out of the race.

Though polls show Missouri Rep. Richard Gephardt leading in the Democratic race and Kansas Sen. Bob Dole leading in the Republican party, the estimated 20 percent of caucus voters who are as yet undecided will cast the crucial votes in tonight's caucus.

Iowa has held caucuses since 1917 because the process is "an effective tool for getting more participants into politics and building grassroots contituents," said John C. Cacciatore, financial director of the Iowa Democratic Party.

A survey of 300 voters likely to attend the caucuses, released Friday by the Cambridge Reports polling firm, suggests a close clustering of support among the Democratic candidates as of the period between January 29 and February 2.

The poll, which has a 5.5 percent margin of error, places Gephardt in the lead with 18 percent, Massachusetts Gov. Michael S. Dukakis second with 16 percent and Illinois Sen. Paul Simon third with 15 percent, said James V. Murphy, chief executive officer of the nationally respected firm.

The poll also gives former Colorado Sen. Gary Hart 10 percent, the Rev. Jesse Jackson 9 percent, former Arizona Gov. Bruce Babbitt 6 percent and Tennessee Sen. Al Gore one percent. The remaining quarter of the voters polled were undecided.

In the Republican party, the same poll placed Dole first with 28 percent and Vice President George Bush second with 25 percent. Former television evangelist Pat Robertson was third with 17 percent, while the rest of the Republican party trailed far behind. New York Rep. Jack Kemp had 8 percent, former Deleware Gov. Pete duPont had 4 percent and General Al Haig had 1 percent and 20 percent were undecided.

Of the estimated 1.6 million voters who will participate in the caucus, attendance will be decisive in the results of the straw poll. A low turnout will help Dukakis, who has maintained fairly constant support, while a large turnout will hurt him, because a surge of enthusiatic voters could come to the polls in support of Gephardt, Murphy said.

Similarly, a low turnout will help Robertson, whose `zealous' constituents will come to the polls under any circumstances, while a large turnout will hurt him by bringing support for all the candidates, Murphy said.

Political analysts said this year has seen the largest media and commercial attention in Iowa and they expect the caucus turnout will be high.

Although only 3 to 15 percent of Iowans participate in the caucus vote, the results are seen as an important measure ofeach campaign's organi zation and thepersuasiveness of its television and printedadvertisments.

With 2487 precincts in 99 counties in thestate, all of the campaigns have depended heavilyon active volunteers to reach all of the voters,while the candidates have tried to make personalcontact with as many Iowans as possible.

Of the Democrats, Babbitt, Dukakis, Gephardtand Simon have very active statewide campaigns,said Cacciatore. Jackson has improved hisorganization since 1984, though it does not equalthe others, he added.

The remaining two hopefuls, Hart and Gore, havelimited their efforts in the state to televisionadvertising.