Democratic National Committee chairman Tom Perez on Monday countered claims by Sen. Bernie Sanders (I., Vt.) and other critics that the DNC did not devote enough resources to last week's special election in Kansas.

Last Tuesday, Republicans won the special election for Kansas' fourth congressional district, a seat that Mike Pompeo vacated earlier this year after he was confirmed as President Trump's director of the Central Intelligence Agency.

Republican Ron Estes won the district with 52.5 percent of the vote.

Some Democrats still declared victory despite losing because the Democratic candidate was closer than in previous elections in Kansas, a historically conservative state. Many were angry at Democratic leaders for not putting more time and money into the race. Sanders on Sunday called out the the Democratic Party for not doing enough in the special election, the first congressional race since Trump became president.

"It is true that the Democratic Party should have put more resources into that election," Sanders said.

The next day, Perez countered Sanders' claim that the Democratic Party did not do enough in Kansas.

In a Monday interview with NPR, Perez was asked if Democrats made a mistake by not putting more money into the Kansas congressional race.

Q: There was an election in a red state just last week. Democrats made a decision not to put a lot of money into the Kansas House race, and then Democrat James Thompson only just narrowly lost. Was that a mistake? Perez: Well, we actually, we invested in the following ways. When people were out there knocking on doors, they were using the DNC's voter file. We were monitoring the election very closely with the state party, we did robocalls at their request. And to me, what's remarkable about that race is that is a beet red district–Republicans usually win by about 30 to 35 points. They won by single digits. If we replicate success like that everywhere, we will flip the House in 2018. And we're making tremendous progress.

Perez and Sanders start their national speaking tour together on Monday. The "unity tour" is set to target all 50 states, and the two will speak out against Trump's agenda. The tour is designed to help unite two wings of the Democratic party, with Perez representing the so-called establishment wing of the party and Sanders representing the more liberal wing.

Back in February, Perez became the chairman of the DNC after beating Rep. Keith Ellison (D., Minn.), who Sanders endorsed for DNC chair.