Flush with cash from a strong fund-raising quarter, Democratic Congresswoman Kyrsten Sinema will launch a four-week TV ad campaign Monday to bolster her bid for the U.S. Senate.

The three-term Phoenix congresswoman is making the largest ad buy to date in a widely watched race that could decide control of the U.S. Senate next fall.

The new Sinema TV ad expands on a shorter digital ad that pairs Sinema with her Marine veteran brother Paul Sheldon, a Tucson police officer.

WATCH Kyrsten Sinema's first TV ad for Senate campaign

The ad’s themes reflect Sinema’s shift toward the center in Congress: Sinema is an independent who “fights for veterans” and is a “friend of law enforcement.”

The ad buy is more than $300,000, according to documents filed by the Sinema campaign.

Arizona has not sent a Democrat to the U.S. Senate in 30 years, since Sen. Dennis DeConcini’s re-election in 1988.

GOP Sen. Jeff Flake’s unexpected retirement after one term left his seat open this fall.

There is also a possibility that the state’s other Senate seat could open up.

Sen. John McCain has been battling an aggressive form of brain cancer for nine months. The median survival rate is 14 months.

If McCain were to leave office before May 30, there would be a primary for his seat in August and a general election in November.

Sinema’s Senate campaign reported raising $2.5 million in the first three months of this year, and has nearly $6.7 million in cash on hand.

Candidates must file their campaign-finance reports for the first quarter with the Federal Election Commission by April 15.

Sinema’s most active opponent in the Democratic primary, attorney Deedra Abboud, had less than $2,000 in cash on hand at the end of December.

On the Republican side, former state legislator Kelli Ward, in her second run for the Senate in three years, raised $1.5 million by the end of last year and had $350,000 in cash on hand.

Ward’s leading GOP opponents, Congresswoman Martha McSally of Tucson and former Maricopa County Sheriff Joe Arpaio, announced their campaigns in January. Their campaign finance reports this month will be their first.

McSally aired a two-week TV campaign in February after her campaign announcement.