The sixth House Republican from Texas announced Monday that he will not run for re-election in 2020 as an unfolding “Texodus” marks a shift in American national politics as Democrats eye the traditionally red Lone Star as potential battleground territory.

Rep. Mac Thornberry, who represented a district in the northern Texas panhandle, said in a press release Monday, quoting a verse from Ecclesiastes: "We are reminded ... that 'for everything there is a season,' and I believe that the time has come for a change. Therefore, I will not be a candidate for reelection in the 2020 election."

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He served 13 terms in Congress since he was first elected to the U.S. House in 1994. As chairman of the House Armed Services Committee, Thornberry helped rebuild the military and reform the Pentagon. During the first two years of the Trump administration, billions more dollars flowed into the defense budget.

Thornberry was one of the longest-serving representatives on either party in Congress, the Dallas Morning News reported. His impending departure marks the sixth Texas Republican in Congress since July to announce that they will not seek reelection.

Rep. Pete Olson started the trend, followed by Reps. Mike Conway, Will Hurd, Kenny Marchant and Bill Flores. Conway is the top Republican on the House Agriculture Committee and Hurd is the only African American Republican in Congress, according to NPR.

"While we steadily invest in the Lone Star State, Washington Republicans just flew into Texas to declare they'll win back the majority and jetted away without a plan to stop the Texodus," Lucinda Guinn, executive director of the Democratic Congressional Campaign Committee, told the Dallas Morning News.

Because Thornberry’s district is strongly Republican---President Trump won by an 80 percent margin there in 2016—the congressman’s resignation does not pose a risk to the GOP losing the seat to the Dems come 2020, the Dallas Morning News reported. Some of the other seats are more vulnerable to a Democratic takeover come Election Day.

Unlike Democrats, the Republican Party sets term limits on how long representatives can hold committee leadership positions in the House. Some speculate Thornberry and other Republican representatives decided against re-election because they don’t want to return to the status of a rank-and-file member of Congress. The Republican Party is considering amending that rule to prevent others from flying to coop, according to Politico.

Others believe Republicans in Congress no longer want to serve in a chamber as a member of the minority party. It’s unlikely the GOP will regain the House as a result of the 2020 election.

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According to the Texas Tribune Washington bureau chief, Abby Livingston, some GOP Republicans might have been dissuaded by the smaller margins by which they won re-election the last time. Though Texas remains red, the Democrats have gradually been seizing influence in a ground up movement at the state level, as more of the wealthier suburbs in Houston and Dallas are now represented by Democrats, according to NPR.

The recent Texodus comes after the Republican Party lost control of the House for the first time in eight years following the 2018 midterm elections. Two GOP congressmen lost their re-election bids that year while six others announced their retirements in 2018, according to the Dallas Morning News.

Fox News' Lucas Tomlinson contributed to this report.