Now or Later, Royal Court Downstairs, LondonKicking a Dead Horse, Almeida, LondonLipsynch, Barbican, London

Christopher Shinn's play, set on the night of an American presidential election, is a potent examination of liberal credentials

Political theatre does not get much more topical than this. Written by the fast-rising Obie-winner Christopher Shinn, Now or Later is set in the Democrats' hotel-HQ on the night of the American presidential election. So what we have here is, surely, the 2008 US equivalent of David Hare's The Absence of War (famously based on Neil Kinnock's 1992 Labour campaign).

Actually, no. Shinn adopts a strikingly different approach. Besides the fact that his Democratic candidate – Matthew Marsh's burly, white John – isn't obviously modelled on Barack Obama, Now or Later deftly sidesteps the hub of party-political action. The flashbulbs of the media circus are flickering many storeys below while we are tucked away in the bedroom allotted to the President-elect's son. Eddie Redmayne's edgy, privately gay John Jnr is keeping his distance, preferring to hang out with his college buddy Matt (Domhnall Gleeson), a scruffy, grass-roots lefty.

What's electrifying is how this peripheral nook rapidly turns into the really fraught hotspot as a scandal spreads like wildfire on the internet. Blurry photos are suddenly posted of John Jnr at an Ivy League party, dressed as the Prophet Mohamed. A string of political advisers – followed by Marsh himself – tries to persuade the youth to make a public apology. As the tension mounts, they mention riots starting in Pakistan and the possibility of a fatwa. However, after years of feeling suicidally subordinate, John Jnr fiercely defends his autonomy and his principles. He insists that the satiric outfit was validly worn in protest over Muslim and myopic liberal undergrads who – outraged by anti-Islamic cartoons on campus noticeboards – called for restrictions on freedom of expression.

Maybe some of Shinn's initial exchanges sound stiff with politico-academic jargon, and the climactically violent father-son clash is rushed. Essentially, though, Now or Later is as potent as David Mamet's Oleanna in its exposure of how liberal creeds can – one way or another – be murkily tied in with intolerance. Shinn reworks diverse real-life headlines into a gripping dramatic knot. Dominic Cooke's cast is terrifically sharp on ulterior motives and the treacherous ground between genuinely caring chats and politic manoeuvres. The spiky intelligence of the arguments is riveting, and the ramifications become hugely scary all within the confines of a chamber piece.

Sam Shepard's previous London premiere – The God of Hell in 2005 – was a nightmarish political satire where a zealous door-to-door lunatic, hardselling stars-and-stripes cookies, tortured any suspected antagonists. Maybe Kicking a Dead Horse – the sexagenarian's new self-directed one-man play – is partly meant as a metaphor for contemporary America, too.

Stephen Rea's crotchety Struther is a big-shot art dealer turned wannabe cowboy, stranded in the Badlands with his steed lying belly-up, stiff as a board. Having dug his grave, Struther feels driven to throw in his Wild West hat while having the following exchange with the cynical voice in his head: "The hat can't save you." "But ..." "What?" "The history ..." "Gone."

Struther's fits of schizophrenic dialogue also touch on destructive colonial pioneers and Crazy Horse as a "true American hero". But if the stetson-trashing is supposed to be some state-of-the-nation symbol, Shepard's point is not clearly articulated. More often Struther looks like a badly disguised portrait of the dramatist struggling with writer's block and the disappointments of old age. There are one or two eerie images, including a young woman rising from the grave like a ghostly memory. But the chat is rambling and Rea's attempts at Beckettian clowning, tripping over his lasso, are embarrassingly feeble.

At his best, the Quebecker deviser-director Robert Lepage is out of this world. His last mercurial one-man show – the Anderson Project in the Barbican's BITE:05 programme – was mesmerising. Now, though, fans should steel themselves because his latest multimedia creation, Lipsynch, is a disappointing blip. A pretty massive blip, in fact, because this baby lasts all of nine hours.

To be sure, this story of a modern-day foundling leaving his adoptive London home to trace the history of his prostituted Nicaraguan mother has both the close-up intimacy and epic sweep of classic Lepage sagas. It maps interlinked lives over decades and continents, and the first hour is superb – visually stunning and hauntingly poignant. Against a skyscape of dark, drifting clouds, a heart-rending operatic aria – ululations of pure pain – is sung by a bereft mother. With a dreamlike fluidity, the giant skeleton of a jumbo jet slides into view and the diva takes her seat in the dimly lit cabin. Amid the perfunctory business of silhouetted stewardesses and sleeping passengers, a baby is crying and, in this otherwise completely mute scene, a whole mini-tragedy is played out – with the singer left cradling the newly orphaned child in her arms.

Alas, this is the aesthetic high point. There are brilliant patches elsewhere, but slow pacing, technical hitches and banal dialogue pull the show down. One cannot help wondering if Lepage has been overbusy on other projects and has left too much in the hands of Lipsynch's less-acclaimed co-producing company, Théâtre Sans Frontières. Half-baked.

'Now or Later' (020-7565 5000) to 18 Oct; 'Kicking a Dead Horse' (020-7359 4404) to 20 Sep; 'Lipsynch' (0845 120 7550) today