By Kim Palmer and Susan Cornwell

COLUMBUS, Ohio/WASHINGTON (Reuters) - Rob Portman is largely ignoring Donald Trump, which is hard to do in U.S. politics today, but the strategy seems to be working for the Republican senator in his re-election bid in Ohio.

As congressional Republicans across the country grapple with the Trump effect on their home-state campaigns, the mild-mannered Portman, 60, may be pointing the way forward in his race, the nation's most expensive Senate contest so far this year.

Last week in Columbus, the state capital, Portman made no mention of Trump in a campaign appearance, though the New York businessman is the Republican presidential nominee and Portman has endorsed him.

On Portman's web site, Trump is absent. Portman has hit the campaign trail in Ohio with a Republican who had White House ambitions, but it's the state's Governor John Kasich, not Trump.

Kasich lost his bid for the party's nomination to Trump. Since then, Kasich pointedly has not endorsed Trump, a property developer and television personality who has never held elected office and whose smash-mouth politics worries many Republicans who fear he may lose them votes in Senate and House of Representatives races.

Those concerns have been compounded by Trump falling behind his Democratic rival Hillary Clinton in opinion polls.

Portman campaign manager Corey Bliss said his boss has never wavered in his support of Trump and there has been no conscious effort to de-couple Portman from the presidential nominee.

But a story on the Cleveland.com web site last week reported that Portman, a veteran Washington insider, has yet to appear at a single Trump campaign event, or even in the same photo, with him.

While effectively disconnecting himself from his party's nominee, Portman has led Democratic rival Ted Strickland by five to nine points in various polls since late July. Strickland is a 75-year-old ex-governor of the Midwestern swing state.

A Monmouth University Poll on Monday gave Portman a lead of 48 percent to 40 percent over Strickland, while Clinton had 43 percent of likely Ohio voters and Trump had 39 percent.

Portman does not view either presidential nominee as crucial to the Ohio election, Bliss said. "We are focused on our race and running for U.S. Senate," he said.

BIG DOLLAR RACE

With the Nov. 8 elections nearing rapidly, the Ohio race is the costliest U.S. Senate show-down in 2016. Candidates, parties and outside groups have spent $45 million, said the Center for Responsive Politics, a campaign finance watchdog group.

Portman worked briefly for former President George H.W. Bush, then was elected to the House. He resigned to be U.S. trade representative in 2005-2006, then director of the U.S. Office of Management and Budget for President George W. Bush.

Strickland, a former Ohio governor, previously represented southeastern Ohio in Congress for 12 years. That region of the state could play a role in the Portman-Strickland fight, but Trump himself is a major factor.

Paul Beck, a political science professor emeritus at Ohio State University, said despite distancing himself from Trump, Portman could still be hurt if Republicans are so turned off by the presidential nominee that they decide not to vote at all.

"These are real worries, and it's a challenge to Republican candidates all over the country, but particularly to Rob Portman," Beck said.

(Additional reporting by Kouichi Shirayanagi in Washington; Editing by Kevin Drawbaugh and Alistair Bell)