He frequently expresses frustration with the Senate's slow pace and partisan gridlock, and he even considered quitting in 2012 to run for his old job as Virginia's governor.
The former cellphone pioneer and venture capitalist is a multi-millionaire - one of the richest members of Congress - and certainly doesn't need a senator's salary to pay the bills.
So why does he want a second term?
Warner, a Democrat, said he's begun to figure out how the Senate works and how he can make it a better place.
Everything I've done in life has not had a straight path to success, it's had pitfalls along the way, Warner said in an interview with The Associated Press. And coupled with that, I refuse to acknowledge the notion that America's not going to continue to be great - and the only way it can be great in our democracy is if you have people who can find common ground.
Warner is facing challenges from Republican Ed Gillespie and Libertarian Robert Sarvis in next month's election. While Warner's Democratic colleagues in several other states are facing tough re-election prospects thanks in part to President Barack Obama's sagging popularity, public polls have shown Warner with substantial leads.
Quentin Kidd, a political science professor at Christopher Newport University, predicts Warner will have a small but comfortable margin of victory over Gillespie thanks in large part to a successful tenure as governor that saw Warner leave the Executive Mansion with favorable ratings.
Mark Warner's job was to go around the state and remind people why they like him, said Kidd.
Warner's healthy lead in the polls has kept the Virginia senate race out of the national spotlight and largely kept away high-spending outside groups that have flooded other Senate contests.
Still, Warner and his supporters aren't taking chances. Warner has raised more than .9 million for his re-election campaign, and an allied super PAC funded largely by a small group of wealthy donors has spent .8 million attacking Gillespie.
I'm pretty obsessed, Warner said of his re-election campaign. We've got a major get out the vote, we've got a great ground game.
Gillespie, a former lobbyist and Republican National Committee chairman, is equally optimistic about his chances and has portrayed Warner as a lock-step Obama supporter whose votes for the president's policies - the Affordable Care Act in particular - have hurt Virginia's economy.
Senator Warner's press releases are very bipartisan, but his floor votes are very party line, Gillespie said at a recent candidate debate.
Warner bristles at the charge. He has frequently distanced himself from the president and said he has several proposals aimed at improving Obama's health care law. Warner also notes that many Virginia Republicans support his re-election campaign, including former Sen. John Warner, who is not related.
Mark Warner frequently points to his work on the budget, where he was part of a bipartisan Gang of Six that tried unsuccessfully to strike a grand bargain on taxes and entitlement programs aimed at cutting the deficit.
This battle will come back, he said. We can't keep kicking the can.
Republican Sen. Saxby Chambliss of Georgia said Warner's bipartisan shtick is legitimate.
Mark has been a very valuable member of the senate, said Chambliss, who said he's become close friends with Warner.
Chambliss added that Warner has grown into his role as a senator and is less frustrated by the slow pace of politics.
In the last couple of years, I've seen a major adjustment on his part that the Senate is what the Senate is, and it just takes time, Chambliss said.
Jim Murray, a close friend and former business associate who speaks frequently with Warner, also said Warner's frustration was waned in recent years. He said Warner gets excited when he finds issues of interest and Republican partners to work with them on.
Those moments have reassured him he might well be able to do some good, Murray said.
Before running for Senate in 2008, Warner flirted with the possibility of running for president. Asked about the 2016 presidential campaign, Warner said he's not interested.
I don't expect that moment to come back, he said. I think I can add value in the United States Senate.
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