Rep. Rod Blum, an Iowa Republican waging a tough battle for re-election, has spent more on taxpayer-funded mass mailings to constituents than any other House representative.

Blum spent more than $400,000 in taxpayer money on mass mailings and mass communications to his district from January of 2017 through March 31, according to expense records reviewed by the Associated Press.

That’s more than any other House member and far more than the three other Iowa congressmen, who spent less than $20,000 combined.

Blum is locked in a heated re-election fight for a third term in Iowa’s 1st District against Democratic challenger, Iowa state representative Abby Finkenauer. The seat is key for Democrats in their campaign to take the House in 2018. Inside Elections with Nathan L. Gonzalez rates the race a Toss-up.

His mailings were funded by taxpayers thanks to a privilege known as “franking.” The privilege, which dates back to 1775 in the United States, allows all members of Congress to send mail to constituents with simply their signature, on the taxpayers’ dime — no postage required.

Blum was also the highest-spending House member on franked mass mailings in 2016, a Roll Call analysis found.

Vulnerable incumbents facing re-election, like Blum, use the franking privilege far more extensively than others.

Legislators facing close races for re-election in 2016 spent almost three-times as much taxpayer dime on mailings than colleagues with safer seats, according to the Roll Call analysis. They also spend more on franked correspondence closer to election season.

Blum said he was proud to be the highest spender on communication with constituents, according to the Associated Press. He also noted he is the top-ranked member for money saved from his office budget, with more than $275,000 in budget savings last year.

Finkenauer criticized Blum’s expenditures as a wasteful use of taxpayer dollars.

“$400K on taxpayer-funded mailings? I know a lot of communities in #ia01 that could do a lot of good with that kind of money,” Finkenauer tweeted Thursday in response to reports about Blum’s spending.

Blum’s office did not respond to Thursday afternoon emailed questions about his mass mailing expenditures.

Franking is intended to allow legislators to communicate information about their legislative duties and constituent services, and federal regulations and House rules impose some limits on franking around elections.

Legislators cannot send franked mail within 90 days of an election in which they are running, nor solicit votes or mention political campaigns in franked mail. Franked material must be reviewed by a House board before it is sent.

But lawmakers can still use franking to send, for example, newsletters touting their legislative accomplishments, press releases about official business and nonpartisan information on voting procedures.

Apart from mail, frankable items also include Facebook advertisements, posters, banners, advertisements and handouts in their district, as long as they are not overt electioneering.

While House members do have a limited budget for franking, there are no limits on how much of that budget can be used on mass mailings.