WASHINGTON — Starting a $75 million national advertising blitz, both parties’ House campaign arms on Tuesday began running television spots that are not expected to relent before November’s elections.

In New York and New Jersey, the Democratic Congressional Committee began spending $2.3 million on ads in districts represented by Republicans. In Florida, the National Republican Congressional Committee started what will be a $1.7 million campaign to defend Rep. Steve Southerland, who risks being voted out of Congress after two terms.

The early spending is only expected to escalate as voters start paying closer attention to elections. Democrats face a steep climb in their bid to oust House Republicans from their majority. Despite long odds, donors have helped Democrats outraise the National Republican Congressional Committee in 16 of the last 18 months and build a mountain of campaign cash to spend on ads against Republicans.

The Democrats’ campaign arm has asked television stations to set aside $44 million in ad time. The Republicans have booked more than $30 million. Both numbers are likely to change as strategists add or nix districts as they become more competitive or slip from reach.

The tone of the first committee ads, of course, was decidedly negative.

“Michael Grimm. Can you believe this guy?” asks the Democrats’ ad against the two-term incumbent Republican facing criminal charges. The 30-second ads are expected to stay on New York City cable stations through Oct. 13, at a cost of almost $1 million.

As black-and-white video of Grimm plays, audio from news reports about his criminal troubles play in the background.

Grimm is accused of evading taxes and hiding more than $1 million in sales and wages from a small Manhattan restaurant. He has been indicted on 20 criminal counts. The Staten Island lawmaker and former FBI agent has pleaded not guilty.

The National Republican Congressional Committee has not booked airtime to defend Grimm and was silent on Tuesday’s anti-Grimm ad. By contrast, the NRCC called Democrats’ ad against its nominee in south-central New Jersey, where Republican Rep. Jon Runyan decided to step down, “despicable and slanderous.”

Burlington County official Aimee Belgard is one of national Democrats’ favorite candidates, and the DCCC is spending $1.3 million on ads through Election Day. The DCCC never mentions Belgard’s name in the first 30-second ad and instead focuses its criticism on the Republican nominee, Tom MacArthur, and his time running a division of insurance giant AIG.

“MacArthur ran an insurance company accused of cheating hurricane and wildfire victims,” the narrator said. “In Congress, MacArthur wants to help insurance companies jack up rates and deny people coverage.”

The accusations about hurricane insurance are particular tough along the New Jersey coastline, which is still recovering from 2012’s Hurricane Sandy. But MacArthur stepped down as chairman and CEO of York Risk Services Group in 2010 and had nothing to do with Hurricane Sandy claims.

Republicans have booked almost $2 million in the Philadelphia media market, which reaches into the New Jersey congressional district. In Florida, Republicans began their own 30-second ads criticizing Gwen Graham, Democrats’ nominee to face Southerland and the daughter of former Sen. Bob Graham.