The Senate Leadership Fund has committed an additional $20 million to Iowa's open Senate race, backing Republican Ashley Hinson against Democrat Josh Turek.

The new spending takes the Republican super PAC's total investment in the race to $53 million. The contest, opened up by the retirement of Senator Joni Ernst, has become one of the most expensive Senate races in the country.

Data from the Washington Post and AdImpact show about $90 million has been spent on the race so far, making it the sixth most expensive Senate contest of the cycle.

Both parties see Iowa as a key battleground in the fight for Senate control. Hinson, a sitting congresswoman, has the backing of the Republican establishment, while Turek is running as the Democratic nominee. The race is expected to draw even more spending before election day.

The race could decide which party controls the Senate next year. Polls show a tight contest, with both campaigns spending heavily on television advertising. Iowa has leaned Republican in recent elections, but Democrats see the open seat as a pickup chance.