WASHINGTON — Independent Republican groups are heavily outspending their cross-party counterparts on television advertising in the campaigns for the White House and control of the Senate, eating into President Barack Obama’s financial advantage over Mitt Romney and prompting expressions of alarm from top congressional Democrats.

The disparity is most evident in the race for the White House, where Crossroads GPS, Restore Our Future and other organizations aligned with the Republicans spent nearly $37 million on TV ads through the first few days of June, most of it attacking Obama. That compares with about $11 million by groups supporting the president, with much of it from Priorities USA Action.

Senate campaigns also have been affected, notably in Ohio, where Democratic Sen. Sherrod Brown’s commanding lead in the polls began to erode this spring after the U.S. Chamber of Commerce and others started a televised attack. Overall, Republican-aligned organizations have spent roughly $30 million on ads in key races, compared with about $11 million for groups supporting Democrats.

Underscoring the concern, Rep. Steve Israel of New York, who heads Democrats’ efforts to regain House control, issued a thinly veiled call for his party’s donors to step up. The recent recall election in Wisconsin “should serve as a wake-up call,” he wrote, referring to the lopsided advantage in spending by outside groups that helped Republican Gov. Scott Walker overcome a union-backed bid to dump him from office.

Other Democratic efforts to catch up are less publicized, particularly when it comes to Priorities USA Action, the group formed to boost Obama’s re-election.

David Axelrod, a top strategist for the president, is expected to meet with potential donors to the group in New York on Monday, according to officials familiar with his plans. Separately, former President Bill Clinton has agreed to help, although it isn’t clear whether he will appear at a formal fundraising event.

Obama’s campaign manager, Jim Messina, and White House aide David Plouffe, who ran the 2008 campaign, met previously with possible donors to the group.

The heavy infusion of outside money comes on top of candidate spending and ads financed by the political parties. While it can alter a race in several ways, one Democrat with long campaign experience cautioned that the impact easily can be overstated.

“You hate to be outspent at all, but in point of fact if you’re communicating loudly and the other guy is communicating twice as loudly, that doesn’t mean he’s communicating twice as effectively,” said Jim Jordan, who has worked in presidential and Senate races.

Outside groups have allowed Romney to remain competitive in the television ads wars while restocking a treasury that was depleted during the battle for the Republican nomination. It also raises the possibility that Obama, the Democratic Party and allied groups will be outspent by a combination of Romney, the GOP and allied organizations, erasing an advantage the president had in 2008.

Earlier this year, Obama’s campaign decided to dip into its own treasury to respond to commercials from the American Energy Alliance, which had spent more than $3 million attacking the president.

Privately, the president’s top campaign aides frequently express concern about the disparity, according to several Democratic officials who spoke on condition of anonymity. There is irony in that, because Obama previously had shunned support from groups that may rely on unlimited or undisclosed donations. It wasn’t until February that he permitted top aides to publicly bless the work of Priorities Action USA.

By contrast, Crossroads GPS and American Crossroads, groups formed by prominent Republican strategist Karl Rove and others, were active in the 2010 election campaign.

Bill Burton, one of the founders of Priorities Action USA, said the pace of donations has picked up as the group expanded its staff and Democratic donors began to focus on a race between the president and Romney. “There’s natural progression,” he said, adding that until recently, it wasn’t clear who Obama’s rival would be, and the imperative they might feel to donate was more distant.

In the Ohio Senate race, ads financed by the Chamber of Commerce, American Crossroads, Sixty Plus and other groups coincided with a drop in Brown’s poll ratings, creating at least the appearance of a competitive race against Republican Josh Mandel where little or none had existed. The most recent attack, airing at a cost of $1 million statewide, says the incumbent voted for “every bailout proposed by (George W.) Bush and Obama,” a rare use of the former president’s image or name in the current campaign.

Brown gained modest support on television from Democratic-aligned groups after he came under attack, and swiftly began using the disparity to appeal for campaign donations.

Brown and many other Democratic incumbents hold advantages over their rivals in campaign cash on hand. But a multimillion-dollar disparity in television advertising by outside groups could eventually stretch the Democratic Senatorial Campaign Committee thin at a time it is struggling to defend a narrow Senate majority.

“It’s a source of great disappointment that people who say they’re on our side remain on the sidelines,” Illinois Sen. Dick Durbin, a member of the Democratic leadership, said recently.

Durbin, Majority Leader Harry Reid of Nevada and Sen. Charles Schumer of New York have all attended fundraisers for the Majority PAC, an organization established to aid Democratic Senate candidates that reported taking in $6.1 million through May 23.

“There’s some movement in our direction but not enough” since then, Durbin added, and other officials said fundraising has picked up in recent weeks.

In the general election race for the White House, television ads designed to aid Obama totaled about $55 million through the early days of June. Of that, the president’s own campaign spent $44.7 million, more than 80 percent of the total, with $9.3 million from Priorities USA Action.

The situation was reversed among Republicans, where outside groups put up about $37 of $44 million spent so far on television ads, or more than 80 percent of the GOP total. Romney’s campaign has spent about $7.8 million.

The Republican advantage comes at a time the system for financing of campaigns is in flux.

Recent Supreme Court rulings mean that corporations and unions may donate unlimited amounts to political groups. The rules are so complex that some organizations must disclose their donors’ names and the amount of their contributions, while others are not. Some television purchases by independent organizations must be reported to the Federal Election Commission with 48 hours, some within 24 hours, and others not at all.