Walsall skipper Andy Butler came to his side's rescue early in the game, clearing off the line from a corner.
Walsall's Milan Lalkovic threatened to capitalise on a weak header back by Aidy White, just failing to lift the ball over goalkeeper George Long.
The home side failed with a strong appeal for a penalty in the latter stages of the half when Lalkovic appeared to trip White inside the area.
Referee Christopher Sarginson did point to the spot just before the break, apparently penalising the visitors for handball and Porter stepped up to blast home the resulting penalty.
Soon after the re-start, Westcarr equalised with a near-post header from James Baxendale's cross.
Walsall's Ashley Hemmings forced a good save from Long with a well-struck shot while United substitute Lyle Taylor's flicked header went narrowly wide.
- Sports & Recreation
- Aidy White
- Craig Westcarr
- Chris Porter