By Roberta Rampton

LAS VEGAS (Reuters) - President Barack Obama on Sunday campaigned in the battleground state of Nevada for Hillary Clinton, the Democratic candidate he wants to succeed him in the White House - but he spent most of his time talking about the state's Senate race.

Democrats badly want to get back control of the Republican-controlled Senate in the Nov. 8 election, and are sending Obama, Michelle Obama and Joe Biden to states where close races could tip the balance.

In Nevada, Obama reserved most of his firepower for mocking three-term Republican U.S. Representative Joe Heck, who had supported his party's presidential candidate until earlier this month when Donald Trump's campaign went into crisis mode by the release of a video in which he lewdly bragged about groping and kissing women.

"I understand Joe Heck now wishes he never said those things about Donald Trump, but they're on tape, they're on the record," Obama said, using Heck's earlier praise of Trump against him.

It's not just the Senate. Obama wants to capitalize on his high approval rating to help elect more Democrats to the House of Representatives, and has also endorsed about 150 candidates in state legislative races.

At an intimate fundraising dinner in La Jolla, California, on Sunday evening, where tickets started at $10,000, Obama urged about 60 guests to help elect Democratic congressional candidate Doug Applegate, a former Marine colonel who is challenging Republican incumbent Darrell Issa.

"As far as I can tell, (Darrell) Issa's primary contribution to the United States Congress has been to obstruct and to waste taxpayer dollars on trumped-up investigations that have led nowhere," Obama said.

Issa, the former head of the House Oversight Committee, led a series of headline-grabbing investigations into Obama's administration - but has featured a photo of Obama signing legislation on a campaign brochure.

"Now that is the definition of chutzpah!" Obama said.

Last week, Obama excoriated Republican Senator Marco Rubio of Florida - who has a narrow lead over Democratic challenger Patrick Murphy, a congressman - for failing to repudiate Trump.

In the final two weeks leading up to Nov. 8, Clinton said she planned to work hard to support congressional and state races.

"We're going to be emphasizing the importance of electing Democrats down the ballot," Clinton told reporters traveling with her on Saturday.

In Las Vegas, Obama was introduced by the Democratic Senate candidate Catherine Cortez Masto, a two-term Nevada attorney general, who would be the first Latina elected to the U.S Senate if she wins.

"We can't elect Hillary and then saddle her with a Congress that is do-nothing, won't even try to do something," Obama said.

The Nevada Senate seat is the only Senate race this year that Republicans could flip to their control. The seat has long been held by Harry Reid, the Senate Democratic leader, who is retiring.

Cortez Masto currently has a slim 2.3 percentage point lead in an average of polls tracked by RealClearPolitics over Heck.

Obama won Nevada in 2008 and 2012. Polls show Clinton with a 4.2 percentage point lead at 45.4 percent support to Trump's 41.3 percent, according to the RealClearPolitics average.

(Additional reporting by Amanda Becker; Editing by Sandra Maler and Chrsitian Schmollinger)