WASHINGTON, Nov. 6 -- The 2018 midterm elections are expected to be the most expensive congressional elections in US history, as candidates for the House and Senate races have raised over 2.5 billion US dollars, according to data released Tuesday by the Center for Responsive Politics.

The organization projected last week that more than 5.2 billion dollars will be spent in this election cycle, making it "the most expensive midterm election ever by a wide margin." This would indicate a 35-percent increase over the 2014 midterm elections, and break the previous record of 4.1 billion dollars in the 2016 congressional race.

The staggering cost of 5.2 billion dollars would include money spent by individual candidates, their political parties and other groups such as PACs, super PACs and nonprofits.

PACs, or political action committees, play a big role in raising fund and advocating for political candidates. Out of the 2.5 billion dollars raised so far for the 2018 House and Senate candidates, 467.4 million, or 18.6 percent, came from the PACs.

Estimates for TV and radio ad spending are around 3.27 billion dollars, according to Advertising Analytics. Estimates for digital ad spending are about 900 million dollars, according to Kantar Media/CMAG.

Democratic candidates have outraised their Republican counterparts both in the House and the Senate, the data from the Center for Responsive Politics showed.

Democratic House candidates have raised 923.3 million dollars as of Tuesday, far ahead of the 612.4 million by the Republican House candidates. In the Senate race, the figure for Democratic hopefuls was 539.9 million, compared to the 395.8 million by Republican candidates.

The large gap of fund raised by candidates of the two parties also reflected strong support for the Democrats, who will have to flip at least 23 Republican-held seats to retake the House in this election. Currently, there are 235 Republicans and 193 Democrats, and seven vacancies.

The Center for Responsive Politics is a nonpartisan, independent and nonprofit research group that tracks money in US politics and its effect on elections and public policy.