WESTON, Wis. - As the bowling ball spun left, missing the lone pin (and a spare) by inches, Rick Santorum grimaced and walked away from the line, muttering to himself about what went wrong and pantomiming his release in an effort to correct it.

"You know he's not going to let us leave until he wins," an aide said. And indeed, Santorum insisted on bowling 10 more frames late Friday evening at Dusty's Weston Lanes, at the end of a long day of campaigning for the Republican presidential nomination.

Santorum, the remaining serious challenger to front-runner Mitt Romney, hopes that persistence pays off Tuesday in the Wisconsin primary, his last chance for the big-state Midwestern win that has eluded him so far. A loss here would reinforce Romney's growing grip on the nomination before April 24, when the contest moves to Pennsylvania, the state Santorum represented for 16 years in Congress.

"We feel we can win this nomination, and we're going to continue to fight," Santorum said.

Romney already has a commanding lead in delegates, and he has reeled in a string of high-profile endorsements in recent days from Republican conservatives as the party's restive leadership steps up calls to end the primary season and get on with the task of defeating President Obama.

Former Florida Gov. Jeb Bush and Sen. Marco Rubio of Florida, the latter a favorite of the tea party movement, have backed Romney, as has U.S. Rep. Paul Ryan of Wisconsin, a budget hawk. "There comes a point where this primary can become counterproductive, where if we keep dragging this thing on, it gets us off the mission," Ryan said Saturday, introducing Romney to about 1,000 activists at a Faith & Freedom Coalition gathering in suburban Milwaukee. Ryan said it was time to "coalesce" around Romney because he would be the best president.

As he has throughout the state, Santorum argued to the same Waukesha audience that Romney is not a true conservative and is "uniquely unqualified" to offer a contrast with Obama on health care, because the federal law, with its individual mandate to buy insurance, was modeled on Romney's Massachusetts plan.

"Don't listen to the pundits and 'experts,' " Santorum said. "They're telling you to give up on your principles in order to win. . . . Stand up for what you know is right for America. Stand up and vote your conscience."

Referring to a recent remark by a Romney adviser that it would take an "act of God" for Santorum to win the nomination, the former Pennsylvania senator said, "I don't know about you, but I believe in acts of God."

Romney has already come from behind in polls to beat Santorum in two Midwestern battlegrounds, Michigan and Ohio, and then crushed the challenger in Illinois. Also on Tuesday, GOP primaries are being held in Maryland and the District of Columbia; Santorum is not even on the ballot in the latter.

After Wisconsin, there is a three-week lull before Pennsylvania votes - the same day as a string of other Northeastern states where Romney is favored, including New York. May brings contests that could be favorable for Santorum, including North Carolina and Texas - if he has enough money to continue the fight that long.

Analysts say that even with a solid win here in the Badger State, it would be difficult for Santorum to prevent Romney from reaching the 1,144 delegates needed to win the nomination, and thus throw open the Tampa convention.

"The Santorum campaign is increasingly sledding uphill at this point in the race," said Josh Putnam, a visiting political science professor at Davidson College in North Carolina who specializes in the primary process. "They are fighting the 'Romney is inevitable' narrative. April is going to be tough, but May offers some hope - just not enough."

And Pennsylvania is no lock for Santorum. A Franklin & Marshall College poll last week found him and Romney in a dead heat there. There has been speculation that Santorum might withdraw from the race rather than face possible humiliation in the state he lost by 17.4 percentage points in his 2006 Senate reelection campaign.

The likely Republican electorate in Wisconsin is shaping up to be similar to those who voted in Michigan, Ohio, and Illinois, though with fewer evangelical voters and more blue-collar voters, two groups that have been crucial to Santorum's success so far.

White evangelical Christians accounted for 38 percent of primary voters in Wisconsin in 2008, compared with 47 percent in Ohio and 39 percent in Michigan earlier this year, according to exit polls. And an NBC News/Marist poll of likely primary voters released Thursday found that about 41 percent of respondents identified themselves as born-again Christians.

According to exit polling, Santorum has not won any primary or caucus to date in which evangelicals cast less than 50 percent of the votes.

"In one sense, Wisconsin is not that distinctive," said John C. McAdams, political scientist at Marquette University in Milwaukee. "The story is we're kind of a typical upper Midwestern state. Given the demographics, and Romney's history of outspending his opponents, you'd have to say he has the upper hand."

Romney and groups allied with him have outspent Santorum forces, 4-1, in television ads alone in Wisconsin.

The NBC/Marist poll showed Romney leading, 40 percent to 33 for Santorum, among likely GOP primary voters, while a poll Wednesday by Marquette's law school had Romney ahead of Santorum, 39 percent to 31.

Complicating the picture in the primary is a recall effort to remove Republican Gov. Scott Walker and key conservative legislators from office, a backlash against their having stripped collective-bargaining rights from public employees. The recall vote is June 5, but under Wisconsin's rules, nominees for that vote will be selected Tuesday.

"We are under assault in this state, and conservatives are more intensely focused on the recall than anything else," said Vicki McKenna, a talk-radio host based in Madison. "That said, I think there's reasonable support firming up for Romney. Santorum has the insurgent grassroots movement, and passionate supporters, but I don't think it's going to be enough."

The Rev. Bob Hoekstra is planning to vote for Romney, though he came to see Santorum on Friday at Loopy's Grill & Saloon in Chippewa Falls, where the candidate spoke to about 100 people gathered under an inflatable dome in the tavern's beach-volleyball facility.

"It's time for the party to tighten up," said Hoekstra, 54, an Episcopal priest and former city councilman and county board member. "Romney would be the strongest candidate in the fall, and I don't like this knuckledheaded stuff I've been hearing from Santorum - that he'd rather vote for Obama than Romney. That's ridiculous."