Demosthenes, For the Megalopolitans
1-2 (tr. Jeremy Trevett; emphasis added):
 Both groups—those speaking in support of the Arcadians
and those speaking in support of the Spartans—seem to me to be
wrong, men of Athens. For they accuse and slander each other,
as if they had come here from one or other of these cities, rather
than being your fellow-citizens and the recipients of these embassies. Their behavior would be appropriate if they were foreigners; but those who propose to offer you advice should be discussing the situation impartially and considering in an amicable spirit
what is best for you.  As it is, if I did not know these men
and if they were not speaking Attic, I think that many people
would suppose some of them to be Arcadian and the others, Spartan.
ἀμφότεροί μοι δοκοῦσιν ἁμαρτάνειν, ὦ ἄνδρες Ἀθηναῖοι, καὶ οἱ τοῖς Ἀρκάσι καὶ οἱ τοῖς Λακεδαιμονίοις συνειρηκότες· ὥσπερ γὰρ ἀφ᾽ ἑκατέρων ἥκοντες, οὐχ ὑμῶν ὄντες πολῖται, πρὸς οὓς ἀμφότεροι πρεσβεύουσι, κατηγοροῦσι καὶ διαβάλλουσιν ἀλλήλους. ἦν δὲ τοῦτο μὲν τῶν ἀφιγμένων ἔργον, τὸ δὲ κοινῶς ὑπὲρ τῶν πραγμάτων λέγειν καὶ τὰ βέλτισθ᾽ ὑπὲρ ὑμῶν σκοπεῖν ἄνευ φιλονικίας τῶν ἐνθάδε συμβουλεύειν ἀξιούντων.  νῦν δ᾽ ἔγωγε, εἴ τις αὐτῶν ἀφέλοι τὸ γιγνώσκεσθαι καὶ τὸ τῇ φωνῇ λέγειν Ἀττικιστί, πολλοὺς ἂν οἶμαι τοὺς μὲν Ἀρκάδας, τοὺς δὲ Λάκωνας αὐτῶν εἶναι νομίσαι.
You should always determine what is right and do it—but at the same time you should
ensure that this coincides with your own interests!
δεῖ δὲ σκοπεῖν μὲν καὶ πράττειν ἀεὶ τὰ δίκαια, συμπαρατηρεῖν δ᾽ ὅπως ἅμα καὶ συμφέροντ᾽ ἔσται ταῦτα.
I would like to hear from those speakers who claim to hate
the Thebans or the Spartans, whether they hate them out of concern for you and your interests or whether they hate the Thebans
out of concern for Sparta, and the Spartans out of concern for
ἡδέως δ᾽ ἂν πυθοίμην τῶν λεγόντων καὶ τοὺς Θηβαίους μισεῖν φασκόντων καὶ τοὺς Λακεδαιμονίους, πότερ᾽ ἑκάτεροι μισοῦσιν, οὓς δὴ μισοῦσιν, ὑπὲρ ὑμῶν καὶ τοῦ συμφέροντος ὑμῖν, ἢ ὑπὲρ Λακεδαιμονίων μὲν Θηβαίους, ὑπὲρ δὲ Θηβαίων Λακεδαιμονίους ἑκάτεροι.
Related post: Sticking Up for Athens