WASHINGTON  — Bernie Sanders has been putting up major advertising cash to seize momentum heading into the Democratic presidential primaries — outspending his rival Hillary Clinton just as voters are beginning to pay attention to the race.

In the past three weeks, Sanders’ campaign has spent about $4.7 million on ads to Clinton’s $3.7 million, an investment that so far has meant 1,000 more Sanders commercials than Clinton ads on broadcast TV, according to advertising tracker Kantar Media’s CMAG.

The Sanders ad burst is coinciding with his rise in preference polls in Iowa and New Hampshire, the first two states where voters will weigh in on the 2016 election. The Democratic contest appears to be tightening.

Two recent surveys suggest Sanders has gained on Clinton in Iowa. In New Hampshire, one poll showed the Vermont senator ahead of the former first lady and secretary of state by double digits, while another pointed to a tighter race. Iowa votes Feb. 1; New Hampshire, Feb. 9.

Clinton began advertising in August, three months ahead of Sanders, and over the entire contest has outspent him by about $3 million, the CMAG data show. The ad buy data includes all broadcast, cable and satellite television, as well as some radio. It does not include digital ads.