Wisconsin Gov. Tommy G. Thompson, who once considered a presidential bid of his own, today will hop on the George W. Bush bandwagon, becoming the 18th of 31 GOP governors to endorse Bush.

Thompson, one of the longest serving and most popular of the nation's governors, will travel to Iowa to announce his support and preside over the opening of the Texas governor's office in the state.

Often mentioned as a possible vice presidential candidate, Thompson dismissed the idea out of hand in an interview yesterday. "That's not going to happen," he said.

Bush "reaches out to different segments of the population that are not always in the GOP fold," Thompson said. "He has been a visionary governor, he is an aggressive leader, and I just plain like him as a friend."

Only Iowans in Straw Vote

Iowa GOP chair Kayne Robinson was in town yesterday to promote his state's Aug. 14 presidential straw poll, and lay down some tight new ground rules for the event. Robinson stressed that non-Iowa residents will be strictly forbidden to participate in the event, which will take place in Ames.

No longer will candidates be able to import busloads of out-of-state supporters to stuff the ballot box, as happened in 1995 when Sen. Phil Gramm (R-Tex.) tied Robert J. Dole for first place in the straw vote.

Robinson said the new residents-only rule will ensure the straw poll is a true test of a campaign's Iowa organization and not simply a gauge of financial prowess.

Nonetheless, candidates still will be able to purchase as many blocks of $25 tickets for supporters as they like, meaning deep pockets will remain a factor and the Iowa GOP will pocket a nice chunk of change regardless of the outcome.

In a breakfast meeting with reporters, Robinson said the straw poll is often an accurate predictor of success in Iowa, whose first-in-the-nation caucuses are scheduled for Feb. 7.

All 11 GOP hopefuls will be listed on the ballot and offered speaking slots, but Bush, among others, has not said whether he will actively participate.

One candidate showing lukewarm interest in the Iowa contest is Sen. John McCain (R-Ariz.). Some Iowa politicians believe McCain may try to skip the state.

GOP Celebrates Converts

Republican National Committee chairman Jim Nicholson went to Florida earlier this week to honor a new milestone for the GOP: the 400th elected official to abandon the Democratic Party and become a Republican since President Clinton was sworn in to the presidency.

The politician selected for this honor was City Councilman Price Wilson, of Chipley. The switch had the added benefit that Wilson is African American and -- Republicans say -- reflects the growing black discontent with the Democratic Party in the state.

"I can remember wondering, `Will we ever reach 300 while I'm chairman,' and today we've gone north of 400," Nicholson said. In fact, a total of 13 Democrats in the Florida Panhandle switched parties this week to bring the total to 408.

Trail Notes

The front-loading of the primary calendar continues. Both houses of the Michigan legislature have approved a bill moving the state's GOP primary up to next Feb. 22, right on the heels of first-in-the-nation New Hampshire. Michigan Gov. John Engler (R) is expected to sign the primary bill into law, the Associated Press reported. . . . John McCain may be thinking of skipping Iowa but he is clearly serious about keeping his campaign coffers full. McCain's campaign yesterday announced that Georgette Mosbacher, a prodigious GOP fund-raiser, businesswoman and socialite, would join the campaign as a national co-chairman.

Staff writer Thomas B. Edsall contributed to this report.

CAPTION: Republican Gov. Tommy Thompson joins 17 others.