In his second round of endorsements, former President Barack Obama threw his weight behind Kyrsten Sinema in Arizona's marquee race for the U.S. Senate in the upcoming midterm elections.

Obama, who is widely popular within the Democratic Party, described his endorsements, numbering 260, as "Americans who aren't just running against something, but for something."

The former president, also waded into Arizona's race for governor, throwing his support to Democratic gubernatorial nominee David Garcia, an Arizona State University professor, and Democratic attorney general nominee January Contreras. Garcia and Contreras are challenging incumbent Republicans Gov. Doug Ducey and Attroney General Mark Brnovich, who are favored to win.

Endorsements are typically more effective in primary elections. Obama's backing likely won't sway a significant number of voters.

"We’re so partisan today, people already know which way they’re going," said Larry Sabato, the political scientist who directs the University of Virginia's Center for Politics and has studied endorsements.

Sinema's spokesman, James Owens, credited the congresswoman's "independent record" for garnering bipartisan support from Obama and former Republican state attorney general Grant Woods. The former attorney general also served as the late U.S. Sen. John McCain's chief of staff in the 1980s during McCain's pre-Senate days in the U.S. House of Representatives.

In a statement, Owens said their enthusiasm mirrors what we’re hearing from voters across the state."

Obama's post-presidential life has been fairly low key in the era of President Donald Trump. But he recently re-engaged politically with his first-round of endorsements, fundraisers and campaign speeches for Democrats.

Obama's office announced his midterm endorsements for governors, U.S. Senate, U.S. House of Representatives and state legislatures across the U.S.

"Our incredible array of candidates up and down the ticket, all across the country, make up a movement of citizens who are younger, more diverse, more female than ever before,” a statement from Obama said. "They’re Americans who aren’t just running against something, but for something. They’re running to expand opportunity and restore the honor and compassion that should be the essence of public service.

"I’m proud to endorse so many of them today, and I’m eager to continue making the case for why they deserve our votes this November.”

Sinema, a three-term Democrat from Phoenix, is vying against Republican Rep. Martha McSally, a two-term lawmaker from Tucson.

Arizona's Senate race is deemed a toss-up by political analysts and is a crucial pickup seat for Democrats hoping to retake control of the Senate.

The candidates and their allies have waged a multi-million dollar fight to win the seat, with Republicans counting on their spending edge to lower the height of the so-called "blue wave" of Democratic enthusiasm sweeping the races across the nation.

McSally is endorsed by President Donald Trump and has leaned into his support since the earliest days of her bid for the seat. Trump carried Arizona in 2016 by about 4 percentage points over Democratic presidential nominee Hillary Clinton.

McSally's campaign spokeswoman, Torunn Sinclair, used Obama's endorsement to cast Sinema as a "proud Obama liberal" Democrat and called her Obama's No. 1 fan, "even helping him shape the disastrous ObamaCare law."

Added Sinclair: "This endorsement shatters the fake charade of Kyrsten Sinema as an 'independent.' "

In a red-leaning state, Sinema has positioned herself as a conservative Democrat who will work across party lines.

Sabato said the presidential endorsements for both candidates could actually repel voters.

"They could say, 'I don't know what to think, but if Obama likes her, I'm going to the other candidate,' " Sabato said. "And the reverse could happen with Trump's endorsement of McSally."

In a statement, Garcia said he was "grateful" for Obama's support.

“President Obama is one of the great leaders of our time," a statement from Garcia's campaign said.

Contreras said in a statement she, too, was "deeply grateful" for Obama's support.

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