Jon Stewart, the arch-liberal US news satirist, is planning a "Rally to Restore Sanity" in Washington next month to draw voters to an anti-extremism demonstration sold on witty irony. He's calling it the Million Moderate March.

His tongue may be in his cheek but it doesn't muffle the star TV host's rallying cry to an exasperated mainstream just days before the midterm elections in November.

Sent up as "a few hours of fun" but in reality a serious riposte to the rightwing Tea Party movement now stealing the spotlight, Stewart promised to supply signs declaring "I Disagree With You, But I'm Pretty Sure You're Not Hitler" and other deadpan slogans.

The day after he announced the rally on his political satire programme, The Daily Show, mainstream news channels were calling the Washington DC police to check it wasn't a hoax.

They were told Stewart had, indeed, applied for permits for a public gathering on 30 October.

It will take place on the National Mall below the Lincoln memorial, site of so many historic demonstrations over the decades. Not least of which was the recent "Restoring Honor" rally organised by Stewart's antithesis, the rightwing conservative Christian TV host Glenn Beck. The "Million Moderate" reference is also a poke at Beck's predominantly white rally, which talked of "reclaiming" civil rights on the anniversary of Martin Luther King's "I have a dream" speech, and in the place also where African Americans held their Million Man March for stronger rights in 1995.

Stewart's event is designed is to counter what he called a minority of 15% or 20% of the country that has dominated the national political discussion with extreme rhetoric.

News of Stewart's rally came at the end of an extraordinary week. It began with a narrowly avoided Qur'an-burning on the anniversary of the 11 September terrorist attacks and ended with former Alaska governor and rightwing darling Sarah Palin stoking suspicions that she will run for the White House in 2012.

In the middle came the giant Tea Party upset, where inexperienced but blowhard conservatives – opposed by their own official Republican party – won primary elections and will now fight for congressional seats in the midterms.

They will not succeed if Stewart has his way. He is even appearing on The O'Reilly Factor, the Fox News show of another of his bellowing rightwing rivals, Bill O'Reilly, on Wednesday.

The last time Stewart went on Fox, he managed to broadcast more criticism of the channel on the actual channel than anyone could remember, accusing it of being a "cyclonic perpetual emotion machine" that has "taken reasonable concerns about this president and this economy, and turned it into a full-fledged panic about the next coming of Chairman Mao".

Stewart is a master of comedy but he is also seriously influential. When he blasted CNN's dog-pit-style political pundit-fight Crossfire programme back in 2004, saying it was "hurting America" with its mindless, partisan bickering, the show was eventually cancelled, with Stewart's criticism cited as one of the reasons.

The largest segment of Stewart's audience is under 30 and has liberal views – and gets as much of its news from The Daily Show as from the evening news programmes and cable news channels.

Stewart's rally will be counter-balanced by a spoof "extremist conservative" rally called the "March to Keep Fear Alive" by his fellow Comedy Central satirist Stephen Colbert.

It may not be enough to help Barack Obama retain control over the lower House of Representatives in Congress, however. The Republicans are firm favourites to wrest the majority from the Democrats.

However, Republican hopes to win control of the upper house, the Senate, too took a significant step back after the success of fringe rightwing candidates in the primary elections last week.

The victory of evangelical rightwinger Christine O'Donnell in the Delaware primary spurred analysts to predict that the Democratic candidate will now beat the original odds and win that Senate seat comfortably in November. O'Donnell is now 11 points behind in the polls in that state, whereas Mike Castle, the moderate Republican she beat last Tuesday, had been strongly ahead.