President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE sought to boost Sen. Martha McSally Martha Elizabeth McSallyThe Hill's Campaign Report: Presidential polls tighten weeks out from Election Day Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' New ABC/WaPost poll finds Trump edging Biden in Arizona, Florida MORE (R-Ariz.) with a Twitter shout out Tuesday as the Arizona Republican faces a tough reelection battle against astronaut and gun control activist Mark Kelly.

“Just signed a critical bill to formalize drought contingency plans for the Colorado River. Thanks to @SenMcSallyAZ for getting it done. Big deal for Arizona!” Trump tweeted Tuesday that McSally retweeted.

Just signed a critical bill to formalize drought contingency plans for the Colorado River. Thanks to @SenMcSallyAZ for getting it done. Big deal for Arizona! — Donald J. Trump (@realDonaldTrump) April 16, 2019

The law Trump signed codifies an agreement between Arizona, Colorado, Wyoming, Utah, California, New Mexico and Nevada to establish voluntary water conservation measures as the states try to manage prolonged drought conditions and regional impacts of climate change.

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The Arizona Republican is running next year to serve the remaining two years of the late Sen. John McCain John Sidney McCainThe Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' Cindy McCain: Trump allegedly calling war dead 'losers' was 'pretty much' last straw before Biden endorsement MORE’s (R) term. McSally was appointed to the Senate in December after McCain’s death in August, with former Sen. Jon Kyl (R) serving in the interim months. However, she was defeated by Democrat Kyrsten Sinema in the 2018 race to replace retiring Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden MORE (R).

Sinema’s victory made the Grand Canyon State a top priority for Democrats, who have successfully recruited a top-tier challenger in Kelly.

Kelly announced earlier this month he raised a staggering $4 million in the first quarter of 2019, with the average online contribution amounting to $25. The Arizona Democrat also has high name recognition as a former astronaut and husband to former Rep. Gabrielle Giffords (D-Ariz.), who narrowly survived a shooting in 2011. The two have since become high-profile advocates for gun control reform.

McSally hauled in about $2 million in the first quarter of 2019, according to a Federal Election Commission filing.

Republicans hope to defend their 53-47 Senate majority next year as a handful of GOP incumbents face tough Democratic challengers in states like Colorado, Maine, North Carolina, Iowa and Arizona.