DES MOINES, JAN. 17 -- Sen. Robert J. Dole (R-Kan.) has increased his lead over Vice President Bush for the Republican presidential nomination in Iowa, according to a poll published today.

The survey of 409 Iowa Republicans who said they were planning to participate in the state's Feb. 8 caucuses indicates that Dole is the first choice of 41 percent and Bush 26 percent. In a similar poll taken Dec. 27, Dole had 37 percent and Bush 33 percent.

{An NBC News poll showed similar results Friday, with Dole leading Bush 42 percent to 26 percent.}

Former television evangelist Pat Robertson moved up from 8 percent to 11 percent. Rep. Jack Kemp (R-N.Y.), with 8 percent, and former Delaware governor Pierre S. (Pete) du Pont IV, with 4 percent, each slipped one percentage point.

Former secretary of state Alexander M. Haig Jr. had one percent, and 9 percent of the caucus-goers said they were undecided.

The poll conducted for the Des Moines Register had a margin of error of 4.8 percent, indicating little significance in the changes other than the widening gap between Dole and Bush, which increased from 4 percentage points to 15.

The poll, taken after the Jan. 8 GOP debate in Des Moines, found that 45 percent of the Republican caucus-goers believe the vice president should be forthright about what he told President Reagan concerning the Iran-contra affair. Thirty percent said it would be improper for Bush to say what he told the president.

Robertson leads the negative column, with 38 percent saying he is the Republican candidate they would least likely support.