HARTFORD, Conn. (AP) -- Connecticut's 5th Congressional District race is expected once again to be an expensive battle, attracting millions of dollars in outside money.

Both the Democratic freshman incumbent, U.S. Rep. Elizabeth Esty, and her Republican businessman opponent, Mark Greenberg, have already been targeted by their national parties as candidates that potential donors should watch. National Democratic groups have already reserved more than $1 million in TV ads to help Esty retain her western Connecticut seat, while Greenberg has already loaned his campaign more than $600,000 and has the resources to contribute even more.

"It's not going to be cheap," Esty said. "We're in an expensive media market. I've got a wealthy self-funder. He has basically a limitless checkbook. We've seen in the past what that looks like in this state, and I expect to see a lot of that again."

Greenberg contends he doesn't think a lot of money should be spent on the race - by him or others. On Saturday, he publicly challenged Esty to join him in asking all third parties not to spend millions on a bevy of TV ads and mailers. If the groups do spend money, one candidate would contribute the amount to the other candidate's charity of choice.

"I just think fundamentally when we spend $3 (million) to $4 (million) to $5 million on each side for a congressional race every two years, there's something wrong," Greenberg said.

Greenberg's challenge to Esty is similar to what has become known as the People's Pledge, or what Greenberg is calling a transparency pledge. The People's Pledge was created during the 2012 Massachusetts U.S. Senate race between Republican incumbent Scott Brown and Democratic challenger Elizabeth Warren to discourage attack ads funded by outside groups.

While the Massachusetts pledge did help to block TV, radio and Internet ads by outside groups, both campaigns still spent a total of $21.7 million. It ranked fourth on the list of spending in U.S. Senate races that year, according to the Center for Responsive Politics.

Bill Evans, Greenberg's campaign manager, said his candidate is open to any kind of limits on spending.

"We're literally putting it out there as an idea and they can come back at us," he said. "If they want to, make it let's cap the race at a certain point. We're open to suggestions."

It appears unlikely Esty will accept the challenge given the continued high level of interest from both sides in winning this seat, considered the most politically equal among the five U.S. House seats. Two years ago in her victory speech, Esty spoke about being "up against the odds" when three out-of-state super PACS opposing her candidacy spent about $2.5 million in the final weeks. She said her victory proved "Connecticut cannot be bought." This year, she's also voiced concerns about Greenberg's ability to tap his personal wealth.

"This is just a deceptive ploy from an ultra-rich tea party candidate who's been running for Congress for six years and has already spent $3 million of his personal fortune in two failed attempts," said Esty campaign spokeswoman Laura Maloney. "How can voters trust him to keep his word when he's already pledged to spend whatever it takes to get elected?"

Esty ultimately outspent her 2012 GOP primary opponent, former state Sen. Andrew Roraback. She spent $3.2 million compared to his $1.57 million and won 52 percent of the vote. Esty loaned her campaign more than $600,000 that year.

As of June 30, Esty's campaign had $1.48 million in cash on hand. Of the money she has raised, $647,614 came from committees, such as political action groups. Greenberg's campaign reported having $263,768 on hand. He had raised $7,500 from committees. Greenberg said he's prepared to spend more of his own money if necessary.

Both candidates say they agree the 2010 U.S. Supreme Court decision concerning the regulation of campaign spending by corporations has been harmful, leading to more special interest money being spent on elections. Esty said she supports comprehensive campaign finance, including a "constitutional amendment that prevents special interest money from drowning out the voices of voters." Greenberg, meanwhile, said he believes U.S. House members should serve four years rather than two, and be limited to two terms. He said that could help limit the influence of outside money.