COLUMBUS, Ohio — Former Republican presidential candidate John McCain offered his first U.S. Senate endorsement of this election cycle, coming to Ohio to throw his support behind fellow military veteran Josh Mandel in a campaign heavily supported by GOP national interests.

McCain said Monday that Mandel, a 34-year-old state treasurer seeking to unseat Democratic incumbent Sherrod Brown, "represents the new generation of leadership" needed to control spending, improve the economy and boost national security.

"Josh Mandel believes in integrity, he believes in the careful stewardship of the taxpayer's dollar," McCain said during an event at the VFW in Hilliard.

Political heavyweights and lots of cash from outside Ohio are rolling into the critical battleground state ahead of what will be one of the nation's most watched U.S. Senate races. New Jersey Gov. Chris Christie endorsed and raised money for Mandel outside Cleveland on Monday, as Sen. Marco Rubio had done earlier.

The early visits by well-known Republicans will serve as a boost to Mandel, who doesn't have nearly the name recognition of Brown, who was first elected to Congress in 1992 and served in statewide office and at the Statehouse before that.

Brown's campaign criticized Mandel for lax attention to his job, including missing the first 14 meetings of a powerful deposit board he chairs and traveling on a weekday to the Bahamas for a Senate fundraiser.

"Josh Mandel can bring in one big name Republican after another and raise as much money as he wants from Washington insiders and special interests, but it won't change the fact that he's not doing his job, can't be trusted, and isn't on the side of Ohio's middle class," said campaign spokeswoman Sadie Weiner.

Both campaigns have been impressive in their fundraising ability so far, leaning heavily on donors and outside groups beyond Ohio's borders. Brown raised $6.5 million through mid-February, while Mandel raised $5.8 million over the same period, according to campaign finance reports reviewed by The Associated Press.

Mandel had collected well over half of his individual contributions — about $2.8 million — from people outside Ohio during that period.

Most of Brown's individual contributions during the same period came from in-state donors, but he also received nearly $1 million from political action committees based outside the state, including many national labor unions.

Brown's campaign said last week it had $6.3 million on hand following the first three months of this year. Mandel had not yet released his latest campaign finance reports, which are due this week.

Those totals don't include money coming in from outside groups, such as former Bush strategist Karl Rove's GPS Crossroads, which is running ads favoring Mandel.

Brown's campaign sent out a fundraising letter last week telling supporters that special interests have spent $5 million on attack ads against them. "We are facing the largest special interest spending gap of any Democratic campaign in America," the letter said.

McCain, a U.S. senator from Arizona who lost his bid for the White House in 2008 to President Barack Obama, praised Mandel's character on Monday and defended him against criticism he's neglected his state job to pursue higher office.

He said outside rating agencies validate Mandel's handling of the state treasury.

"As long as he's doing his job, and the judgment of his peers is that he's doing an excellent job, then I don't think the voters are concerned about that," McCain told The AP after an endorsement event. "They recognize that you've got a campaign (going on). I'm sure Sen. Brown will be missing votes this September and October as he campaigns for his own re-election."

McCain said Mandel's two tours of duty in Iraq as a Marine make up for his youth and relative political inexperience. Mandel was elected to the Lyndhurst City Council at age 26 and later served in the state House for four years before he was elected treasurer in 2010.

"I found in my own case, and that of my son Jim, there's very few things as maturing as serving in combat," McCain said.

Associated Press writer John Seewer in Toledo contributed to this report.