Cranston, R.I., Mayor Stephen Laffey is expected to announce this week that he will challenge Sen. Lincoln Chafee in the GOP primary next year, Rhode Island Republicans said.

Laffey’s entry into the race complicates Chafee’s reelection prospects and enhances Democrats’ prospects of picking up the seat in the overwhelmingly blue state, the GOP officials added.

“We don’t like Republican primaries,” said state Senate Minority Leader Dennis Algiere (R), who backs Chafee. “It could divide the party, and I’m hoping that will not be the case.”

Republicans in Washington for the past several months have sought to keep Laffey from challenging Chafee. Senate Majority Whip Mitch McConnell (R-Ky.) attended a fundraiser earlier this year for the Rhode Island senator. Senate Majority Leader Bill Frist (R-Tenn.) and Sen. Elizabeth Dole (R-N.C.), chairwoman of the National Republican Senatorial Committee, have consistently made it clear they would do everything possible to help Chafee win a second term.

The first sign that Laffey was on the verge of jumping into the race came over the weekend, when he e-mailed supporters and journalists, letting them know he would be making a “major announcement” Thursday about his “future plans” at a Knights of Columbus hall in Cranston.

While politicians often issue brief statements via e-mail or fax to say they are not entering a race, Laffey’s statement indicated the hall would open at 5:15 p.m. and the announcement would come 45 minutes later, presumably to give a large crowd time to show up. (One Republican said Laffey was hoping for 3,000 attendees.)

Algiere said he had heard “unconfirmed rumors” that Laffey would be announcing his Senate candidacy. Other Republicans privately predicted that Laffey is mobilizing for a bid.

“It sounds like he’s going to do it,” a senior Rhode Island Republican official said.

Chafee spokesman Steve Hourihan said simply: “We always expected there would be a Republican primary of some sort.”

Laffey has long made it known that he’d like to run for Congress — against Rep. James Langevin (D-R.I.), Sen. Jack Reed (D-R.I.) or Chafee, a centrist who has alienated many in his party by bucking, at times, President Bush on tax cuts and the Iraq war.

The senior Rhode Island Republican official noted that hints of a Laffey primary challenge have been out there since last year, when the Cranston mayor helped elect former banker Robert Manning as the Rhode Island GOP national committeman.

In a move late last month that deprived the Chafee campaign of critical campaign funds, Manning blocked a $500,000 contribution from the Republican National Committee to the state GOP. The money would likely have gone toward reelecting the senator.

Manning declined yesterday to take sides in the primary, saying only that he felt it best for Republican voters — not officials in Providence or Washington — to decide who should represent them on Capitol Hill.

“My opinion is that when you have seasoned, proven politicians contesting in a primary that it’s beneficial for the voters,” Manning said.

Pat Toomey, president of the conservative Club for Growth, earlier said he would like to see Chafee challenged and offered praise for Laffey.

The Club’s executive director, David Keating, said yesterday that it remains to be seen if the group will get involved in Rhode Island, but he added, “We’re definitely interested in the race.”

It is unclear if the primary will force Chafee — whose record Keating called “horrible” on “pro-growth tax cuts” — to strike a more conservative tone.

Chafee has not indicated how he will vote on Bush’s nomination of John Roberts to the Supreme Court. NARAL Pro-Choice America, a pro-abortion-rights group that opposes Roberts, has endorsed Chafee.

Democrats, for their part, also are enmeshed in a Senate primary, with former state Attorney General Sheldon Whitehouse running against Secretary of State Matt Brown. Brown entered the race first and, early on, raised a great deal of money, particularly from outside the state, but Whitehouse has the backing of key state Democrats, including the state’s two House members, Langevin and Rep. Patrick Kennedy (D). Many Republicans, including state House Minority Leader Robert Watson, said they expect Whitehouse to win the primary.

Hourihan said Chafee plans to spend $4 million on the race. He would not say how much the senator hopes to raise in the third quarter of this year, which ends Sept. 30.