Yes, but it's not too smart (injury and the like - runners rarely slide into 1st base).
Simply put - a fielder with the ball may legally obstruct any runner in order to make a play on that runner. That is kind of the idea of making a play, if you think about it.
Since 1B is a "force" play, there is no need to tag the runner, so blocking the base is pointless unless the fielder is trying to slow the runner down to give, say, F5 more time to make the long throw. That would be obstruction.
But, if the ball arrives at the same time as the runner, it is legal.
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Tom
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